The present invention relates to the field of microwave antennas which are comprised of waveguide elements. Slots are widely employed as elements in scanning antenna arrays. Many large scanning antennas are composed of a two-dimensional array of rectangular slots, in which each slot is fed by a separate waveguide. These devices, however, require a different size slot and waveguide for each frequency band. Moreover, rows of antenna elements made of hollow waveguides are difficult to stack due to space requirements.